The role of sleep in memory and problem solving

Numerous anecdotes and a few studies suggest a link between sleep and problem solving, showing improved puzzle solving or increased use of a hidden rule after sleep compared to an equal period awake. Capitalizing on similarities with memory processes, we applied the targeted memory reactivation technique to problem solving. During 2 evening sessions, participants attempted to solve puzzles while listening to unique sound cues arbitrarily paired with each puzzle. Each night, we presented half the sound cues during slow-wave sleep. In the morning, participants re-attempted previously unsolved puzzles, and solved reliably more of the puzzles cued during sleep, compared to uncued puzzles. In addition, the degree to which cueing increased participants’ recall of puzzles correlated with increased solving of cued puzzles, but not with uncued puzzles. This suggests that sleep facilitates solving through the same mechanisms by which it facilitates memory. Furthermore, despite all the puzzles being classic insight puzzles, cue-enhanced solving was more often reported to come via analysis than via insight; and analytic- but not insight-solving was correlated with the memory cueing effect. This suggests that not only memory strengthening, but memory reorganization, occurs during sleep and is linked to improved problem solving. We further investigated this by examining puzzles that induced fixation (misdirection) versus those that did not. If memory reactivation during sleep enhances all puzzle memory, puzzles with fixation should be harder to solve; if reactivation helps reorganize memories, then puzzles with fixation should particularly benefit from cueing.